The present invention relates to optical fiber telecommunication systems and to optical fiber amplifiers and optical fiber couplers that are employed in such systems.
Fiber amplifiers, in which useful gain is afforded by the stimulated emission of radiation, typically include a gain fiber 10 (see FIG. 1), the core of which includes active dopant ions. A wavelength division multiplexer (WDM) fiber optic coupler 11 can be used for coupling pump power of wavelength .lambda..sub.p from laser diode 15 and the signal of wavelength .lambda..sub.s from input telecommunication fiber 14 to gain fiber 10. Such devices are disclosed in U.S. Pat. Nos. 4,938,556, 4,941,726, 4,955,025 and 4,959,837, for example. The fiber pigtails extending from coupler 11 are connected to other optical fibers by fusion splices or butt joint connectors, splices being preferred because of their lower reflection and insertion loss.
In the system of FIG. 1, a splice 16 connects the input fiber 14 to coupler fiber 13, and a splice 17 connects gain fiber 10 to coupler fiber 12. For optimal amplifier operation, the input signal splice loss at splice 16 should be small in order to maximize signal-to-noise (S/N) of the amplifier because in the signal-spontaneous beat noise limit, the electrical S/N of the amplifier depends linearly on the optical coupling efficiency. Also, the splice loss between the coupler fiber 12 and the gain fiber 10 should be low for both good coupling efficiency (for the same S/N reason stated above) and pump coupling efficiency since amplifier gain is related to the amount of pump power coupled to the gain fiber.
Commercially available telecommunication fibers typically have mode field diameters (MFDs) in the range of 9 .mu.m to 11 .mu.m for light at 1550 nm and 6 .mu.m to 8 .mu.m for light at 1000 nm. Conventional WDM coupler 11 is typically formed of two matched fibers that have been chosen to minimize the splice loss to such telecommunication fibers. For relative index differences .DELTA. found in typical telecommunication fibers, a splice loss of less than 0.1 dB is obtained when the ratio of MFD's of the two fibers is less than 1:1.05 at 1550 nm and less than 1:1.14 at 1000 nm.
Gain fibers operate best when intensities of both the pump and signal beams are high. This can be accomplished by providing the gain fiber with a relatively small MFD, a characteristic that causes the optical power to be concentrated in a relatively small area along the fiber axis. Such a "high gain" or "high efficiency" fiber can be achieved by employing a relatively large core/clad .DELTA. and a relatively small core diameter. There is no maximum acceptable MFD for high gain fibers; however, the MFDs of such fibers should be smaller than the MFDs of standard telecommunication fibers, that difference preferably being larger than 1.5:1.
The mode field mismatch between small MFD high gain fiber 10 and large MFD fiber 12 causes high insertion losses at splice 17. Consider, for example, a telecommunication system employing an erbium doped gain fiber having MFDs of 6.4 .mu.m and 3.7 .mu.m at 1550 nm and 1000 nm, respectively. The gain fiber is capable of amplifying signals at wavelengths between 1530 and 1560 nm; of the various possible pump wavelengths, 980 nm is preferred. A splice between that gain fiber and a telecommunication fiber having MFDs of 10.5 .mu.m and 5.7 .mu.m at 1550 nm and 1000 nm, respectively, exhibits splice losses of 0.5 dB and 1.7 dB at 1536 nm and 980 nm, respectively. Such splice loss reduces amplifier gain and the useable output power of the amplifier.